The technology uses safe electrostatic or quasistatic electric fields called E-fields. The sensing location occurs within the near field zone of an alternating electromagnetic source. The source may be background signals in passive modes or a generated signal in active mode. E-fields can range from sub-Hertz frequencies to 10s or even 100s of MHz or more depending upon the spatial extent of the sensing system relative to the source field wavelength.
A key characteristic of an E-field system is the source and receiver are separated by a distance much less than a wavelength of the electromagnetic energy, a condition referred to as the near field zone. E-field sensing is different from radio systems because radio relies on generating radiated power that propagates out to infinity in the far field zone. On the other hand, E-field sensing does not need a source that radiates power, it must have a sensor sensitive enough to detect electric fields and relate changes to the situational environment.
The challenge to the design of E-field systems is to gain useful information about an environment due to its electric fields. E-fields sensors can detect radio waves but when used in a system containing near fields source they detect E-fields. E-field sensing technology presented herein is novel because historically, near fields generated by equipment, power lines, and electronics are considered noise when they are produced by sources not intended for radio transmissions.
A critical part of making E-fields sensors is tailoring the frequency response to the desired application and to provide the stable biasing to achieve desired sensitivity. The challenge of E-field sensing has been first aimed at sensing low frequencies or quasistatic fields that required DC coupled sensors such as described by Beatty and Zank et. al.
Cehelnik then taught in U.S. Pat. No. 7,078,911 how to tailoring the frequency response with an equivalent circuit and bias resistor to allow for passive mode decrease in a background signal when the body was in contact with earth-ground.
Cehelnik, next taught in U.S. Pat. No. 7,242,298 that capturing simultaneous E-field signals of DC and AC sensor coupled signal provided addition information in a passive system about a plastic or static charged object held in the hand. Again the body was earth grounded.
Both U.S. Pat. Nos. 7,078,911, and 7,242,298 of these specifications stated the sensors could detect active signals. A difference amplifier was also introduced between a common ground electrode and the active antenna and the common as shown in FIG. 1 of U.S. Pat. No. 7,242,298.
Cehelnik, then taught in U.S. Pat. No. 7,358,742 how an E-field bias electrode is useful and 442 can be used to control the AC sensitivity and DC bias of an E-field sensor. These E-field sensor system in this case were done with the passive case of the body being earth grounded or held at a fixed potential relative to ground.
Still pending, Cehelnik's Ser. No. 11/446,768 Method of Alerting Physical Approach and the provisional patent application 61/070,106 both showed the exploration of sensor configurations used to detect proximity of bodies in operational modes such as that used with portable devices when the body is not earth grounded. These included passive with body earth grounded, passive with body floating or at a fixed potential relative to ground, and active where body is augmented with a potential.
In previously stated references, except for Beaty, the E-fields sensor had a sensing electrode connected to an integrated circuit operational amplifier. Integrated circuits operational amplifiers are not transistor level building blocks, and thus to populate a compact high density array of such sensors is prohibitive in cost, and size. Even though Beaty had a floating gate, the disclosed design is primitive as there was no bias resistors but an LED. The frequency response was not controlled.
In provisional application 61/070,106, Cehelnik shows how to use an active DC signal on a body and control the AC output of the background. A floating input opamp circuit is used. The E-field bias is used to control the operating point of the opamp. The DC offset at the output is monitored and a feedback is used to adjust the voltage on the reference electrode. The reference electrode is the shield of a coax, and the center part is connected to an antenna element.
In provisional application 61/070,106 a discrete transistor JFET E-field sensor and an array cross hatch configuration are described. A difference amplifier is shown at the output of the FET amplifier teaching that by touching the common of the battery operated device with one hand, and moving another body part close to the sensing electrode that the sensors output voltage decreases giving the similar behavior as shown in U.S. Pat. No. 7,078,911 with an earth ground. The reason for this behavior is if one touches the common electrode that is floating and the body is floating, the body conducts the background field and moving the hand the other hand closer to the sensing electrode the signal is seen to increase. The difference amplifier is needed make the measurements relative to the floating body.
In provisional application 61/070,106, and its duplicate predecessor filed around Jun. 11, 2007, a floating gate “Smart-Pad” array in FIG. 4a) item [1] was disclosed on circuit board. It was made of parallel sensor antennas on a double sided circuit board with orthogonal elements on opposite sides and was Cehelnik's first insight to a configuration for compact high density arrays wiring scheme capable of being integrated into computer display technology. Several FET biasing and switching schemes were discussed to allow for a two dimensional array of M+N elements instead of M×N combination like pixels.
However, using the floating gate array circuit of JFETs MPF102 shown in FIG. 4a as the first attempt of making a “Smart Pad” had some problems discovered for after filing making it unsuitable for a MCS systems.
A problem was the bias on the gate eventually floated away causing the output to turn off or clip when it was used away from a computer display. This may not occur as problem when the array is mounted on the display or a large conducting plane at dominating reference control potential. The floating gate FET was indeed successfully used as a sensor to pick up E-fields from a CRT and LCD displays as discussed in 61/070,099, “E-field Imaging and Proximity Detection Using a Spatially and Temporally Modulated Source”. Note, the voltage was +9 V making the gain more than the Smart Pad. The smart pad was being run with a +5V Vdd supply.
One, explanation is the capacitance between the sensor electrode and the reference electrode needs to be significant enough to allow battery level voltages to control the DC bias. When using small sensor electrode near the display monitor, the reference electrode was that of that of the ground plane of the face of the video display. This was observed in 61/070,099 with both an opamp and a discrete JFET E-field sensor having floating sensing electrode. As those sensors moved back from the display 10 inches or more, the reference bias was no longer provided. That is why addition reference plane was used in 61/070,099 to allow for detection further from the display. Also note, the floating gate FET was that of FIG. 3b with the bias resistors attached to the gate removed.
However, another difficulty to be discovered was the sensor was too sensitive to low frequency quasistatic fields like footsteps to make an MCS. This is why as these could be seen during power up before the full sensor saturated, or when touching the sensor electrode to ground then watching. Henceforth came the idea in the future of switching the gate voltage on the sensor element to maintain bias as shown in FIG. 1b) and FIG. 1c). Foot steps and movement of static charge and triboelectric charging of moving objects were detected as DC transients at times move between the voltage rails of 0 to 5 volts. Thus the bias would change with foot steps and the AC amplitude changed. This means two things. One the E-field floating gate amplifier is not linear and also was seen to total turn on and off with foot steps, and the AC gain would change. The nonlinearity is not much of a problem for FM waveforms and is a problem for amplitude dependent communications such as a MCS. The turning off is a problem to receive any kind of modulation.
The floating gate FET without bias control operates mostly a switching amplifier when steps were present. The operating point of the FET changes with the potential on the gate. This helps to explains why Beaty's electrometer had a sensitive DC sensor to foot steps where the E-field from the footsteps would turn on and off the LED.
Note that both 61/070,106 and its predecessor provisional application describe a resistor biased JFET transistor amplifier in FIG. 3b. The subsequent filter and amplifier is made for high frequency detection of E-field horizontal 30 MHz bandwidth video signals from a video display unit such as a CRT or LCD display. At these frequencies the display acts as E-field active source. The signal level does not decrease noticeably like at low audio frequencies when the body was in contact with the common and the hand is approaches the sensor. These technologies are discussed in both provisionals 61/070,099 and 61/070,106. Non-the-less they both show bias resistors were contemplated but not directly pursued for passive MCS systems because bias resistors sacrifices sensitivity and adds wiring complexity. The voltage gain difference was found from the bias MCS sensor to a non-biased IC to be about a factor of 10.
Recognizing the floating gate problem, Cehelnik in 61/070,106 introduced a 2D octagon sensor array for portable use.
At least six practical problems were encountered with the octagon design for making an array for a MCS. One, the frequency response was extremely low, and foot steps and static charge from shirtsleeve set off the sensor and distorted the amplitude response. Two, was the limitation on the microcontroller's analog-to-digital converter (ADC) sample rate and processing speed. We were trying to get all sensor elements sampled and signal power levels measured. To get accurate power levels we needed more than a few cycles of the background 60 Hz, and this meant the update rates were limited to 20 Hz assuming we sampled all 8 elements simultaneously for say 4 cycles of 60 Hz background. This put demands that a low cost processing system of the CY24C784 microcontroller could not meet. Three, the background AC field of a portable system was not necessarily well controlled if the pod or body is near other electrical field sources or ground planes like monitors. Four, the power level was not linear with distance. Four, the size of the array, and the layout was large. Five, there was no real way to scan in angle and direction toward the user. Horizontal scan direction finding could be done as in Zank, et. al. Six, a useful change in range algorithm did not exist, the range signal was not linear.
Thus what is needed is a solution to make a MCS array to overcome the practical limitations of the above paragraphs.
Detailed Discussion of Technology
In this application, Cehelnik teaches how to E-field image and to proximity detect and track in 3D the motion of a body using an array of sensors. Cehelnik's previous patent applications taught how to use a JFET integrated circuit opamp with a buffer configuration to make an E-field sensor. Now we discuss how discrete JFET or MOSFETS transistors are used to facilitate miniaturization and fabrication of arrays of closely spaced E-field sensing elements.
Previously, Cehelnik has shown that JFET input type operational amplifiers having an electrically floating high input impedance mode provides superior E-field sensitivity allowing for improved proximity and imaging. However, to control the gain of the sensor, a DC bias electric field needs to be generated at the sensing element or antenna to control the DC output. The bias electric field is provided by an adjacent electrode or “reference electrode”. This was discussed by Cehelnik the AC & DC coupled E-field sensor application, and in the “Method of Physical Approach”.
Cehelnik has shown in the previous applications that by changing the bias field by disruption or conduction from a proximate body results in a change of gain of the amplifier, and facilitates detection. Alternatively to maintain or control the frequency sensitivity of the amplifier or the overall gain of the amplifier, or both, Cehelnik has shown that by adjusting the bias to the input of the amplifier, we can achieve a desired operating point or gain. Even a reference electrode with a feedback network was shown in the previous applications to facilitate control and maintain sensitivity. It was also shown that it was useful to control the AC gain of the sensor by using the fact that these opamps had a DC offset output related to the bias of the JFET input electrode. In fact, here we state the DC and AC gain depended upon the DC offset.
Cehelnik has stated in the previous applications and again here. It is significant to an E-field sensor sensitivity to remove the bias resistor network to the input of the amplifier to preserve input impedance. However, the input is electrically floating, and there is no device control of the gain, the background DC field sets the gain of the device. The amplifier gain is floating, and can even be turned on or off depending upon what the electric field is near it.
The sensor using the floating electrode arrangement or now called “floating gate” since the gate of the input of the JFET is electrically floating. This configuration is not stable by itself because the gain is not definite when the sensor is moved or the background changes.
Cehelnik repeats here that it is useful to make the amplifier operated in a controllable way, by having a reference electrode that provides an electric field to the sensor input electrode. The reference electrode provides a dominating electric field to the sensor. The floating gate configuration presents the input impedance of the FET device to the sensing electrode. It was shown that the DC output was controlled by an external electric field or bias field produced by a reference electrode. The bias field can be generated by natural fields like that of the earth's electric field at the sensor, or by equipment or a source field proximate the sensor. It was also shown that the AC sensitivity was controlled by the DC output or voltage offset of the amplifier.
Cehelnik has also shown that filter in the front of the buffer amplifier can be made to trade with a floating gate sensor at the expense of sensitivity. However, there are times when the frequency content of the desired signal is best filtered with a bias of the gate with a shunt capacitor, to pass only DC, or a shunt resistor when no reference electrode or constant is available, or the case when a shunt resistor and shunt capacitor are both used to filter the AC entering the amplifier. For example, a 1 uF shunt capacitor at the front end of the MCS sensor results in a DC response of the amplifier and is useful for sensing slow motion creating low frequency fields like foot steps or plastic wand motions. Such a filtering capacitor can also be in a sensor with a buffer amplifier not having a bias resistor.
In previous patent applications, the E-field sensing using opamps were discussed. A difficulty with using the opamps in arrays is the cost, and placement of such opamps in an array. To detect the electric field a precise locations the sensing electrode or antenna needs places at the spatial location. Any extent of the antenna can result in detection of electric field signals at the extended location.
To make arrays it is desirable then to have E-field sensors or buffers located a close to the electrodes as possible. If we embed E-field sensors in a computer screen or finger sensing pad we can see the required level of integration with small parts can easily exclude the use of an opamp chip. Cehelnik has indicated that multiplexing elements with E-fields sensors is of value. It is not obvious though, how to meet the requirements of packaging and performance, and update times.
A problem with multiplexing the antennas is the switches have to preserve the high input impedance of the antenna, and the switches have to be located near the location to be detected. Cehelnik, has discussed and shown that it is possible to shield part of the antenna electrodes where sensing is not desired. This helps in routing wires from sensing electrodes to a multiplexer. However, the multiplexer has to have a high input impedance for sensitivity, and if not, then the detection of the signal on the sensing electrode is greatly diminished.
Generally these type of analog multiplexers do not exist. However, if sensitivity is not a critical driver, for example in touch screens the multiplexer can precede the E-field sensor element. Touching is on a flat panel is a 2D process, and gestures can be detected. Cehelnik, shows here that using a 2D grid we can make a touch or screen by simply multiplexing the inputs of a cross hatched array into an E-field sensor.
Another need is to find a way to switch signals from a multidimensional array of E-field sensor signals. If an opamp is used at the location of the E-field sensor, positive, negative, and ground power lines are needed, along with output line is needed, and the input antennal. This is a total of five wires that need routed to each E-field opamp sensor. An N×M 2 D array has N*M output wires. It quickly becomes prohibitive to wire for high sensor count densities. It is easy to see this is a major problem to achieve a video display type resolutions.
Also, the power consumption is a concern. It is generally necessary to conserve power and extend battery life on portable E-field sensor arrays. In an array, it becomes at time necessary to switch off the sensor amplifiers to reduce heating and power consumption.
Using the least number of sensors in arrays is necessary to reduce complexity and power consumption. Sensor counts of N×M can be too many sensor elements that exceed cost and complexity of the devices. In motional command applications for portable electronics, it is shown herein how to reduce the sensor count to N+M sensors. In imaging applications, we can reduce the number of elements needed by scanning a single element mechanically over a 2D grid of locations, or a 1D line array over a 1D grid of locations perpendicular to the array. However, in a camera type operation a 2D array is required.
To reduce sensor processing circuitry, signals from each element are measured at different times, or a difference measurement is made between elements at different times. This is time division multiplexing where the scan time is divided by the sensing time on each element. For motional command applications, the time of scan of the array sensor elements is related to the duration of the motional command. Typical video refresh rates are sufficient, and even less. For Imaging, the duration of the scan depends upon how long the body is present or can remain stationary.
For a MCS the other issue is to detect the person using an array. Cehelnik has shown by applying a potential to the body the E-field sensor can sense the DC value, and also the AC signal. To detect the position of a finger, stylus or control object or surface, the potential that the sensor input uses as the reference is applied to the body. The current is limited to keep it safe. The DC or AD signal is applied to the body, because the corresponding change in the detected DC or AC signal is used to indicate proximity to an element. These detection and modulation modes of operation were shown with opamps and will be shown herein how to use them with the disclosed discrete FET(s) E-field sensor arrays. To solve the stated problems in array assembly, herein is discloses an apparatus of E-field sensors containing a discrete JFETs or MOSFETs. The discussion also applies to the making and use of single transistor E-field sensors useful where cost or long wire routes are an issue or size is an issue. The output of these sensors are followed by amplifiers, and thus form a complete E-field sensor such as in the opamp version, and all the technology of the previous applications apply to this composite type device. In some instances, the FET sensor provides significant output and thus can go directly to the detection circuitry and signal processing.
This CIP Application Further Describes Invented Sensors and Array
FIG. 12—Describes a more sensitive E-field sensor array implementation that switch electronically the bias connection to FET sensors using an analog multiplexer. It also shows a dual sensor in an array that allows a differential measurement between Output A and Output B to reduce common mode signals. It offers low frequency response with DC sensitivity and better linearity than a fixed gate bias resistance R1 described earlier such as in FIG. 2. The unite tested with no R1 and Vs=0 under switched bias showed 3 dB low frequency cutoff around 3-5 Hz, and high frequency response out to 30 kHz. The upper limit was limited by test equipment, and it is reasonable it extends into 100 kHz and even MHz.
The switch time allowed R1 to connect to sensor A and B for 20 ms. Then it held them both disconnected from R1 for several seconds. The time intervals are easily shorted or increased as needed by the designer. After the connection is made there is a transient for 100 microseconds or so that is low frequency. Waiting after this transient to sample the signal is an option for low frequency analysis, but make little difference for high frequency signal in audio range and above.
The switch bias mode has best gain essentially independent of the value of R1. Thus zero Ohms is simplest implementation, and VS is at common ground potential. However, we used R1 at 10 MOhm to allow us to stop the switching an have a steady state connected R1. Zero Ohms works best from a parts count perspective. The gain is 7-10 dB more than a connected R1, and the signal to noise ratio increase 5-8 dB. There is insignificant 60 Hz intermodulation that are present when a connect R1 is used.
FIG. 13—Shows Low frequency response of 36 Hz signal from a hand located 4 inches away indicated by marker A obtained with a 128 point FFT for outputs A and B from FIG. 12 sampled directly into a differential 14 bit Analog to digital converter of a Cypress Semiconductor PSOC CY8C24994. The sample rate was 121 Hz. The ordinate is in dB, and abscissa is in Hz.
FIG. 14—Shows cross array elements for sensor A and sensor B to measure locations of a hand or body relative to sensor panel. 1 shows front view, and 2 shows top view. C is a reference element or plane that allows sensor A and sensor B to measure their output relative to an E-fields sensor connected to element C. The gain in each channel A, B, and C, can be adjusted for size of elements, and signal processing is used to balance the measurements of A−B and a representation of the average of A and B, such as A only or some other metric. Thus we are presenting a proximity sensor that detects X,Y of the body that has a source signal either generated from a voltage modulator held by the person or near by that couples to the persons body or a received from the background signals, such as fluorescent lights, main lines signals 50-60 Hz, or monitor refresh signals, or radio. The elements in A array and B array are switched to find the elements having the larger signal. These elements locations correspond to the X,Y intercept point for the body. The Proximity or range to the sensor array from the body are indicated by the range metric or signal strength on both elements. Changes in range, and X, and Y positions with respect to time indicate speed and velocity that are easily measured by the time between measurement updates.